Natural History of Sharks, Skates, and Rays

Early Chondrichthyes

MARE 380Dr. Turner

Defining ElasmobranchsWhat kind of Thyes?...Chondrichthyes

What kind of Fish?...Cartilagenous Fish

What kind of Brates?...Vertebrates…

What kind of Dates?...Chordates…

Phylum Chordata3 subphylum:Subphylum Tunicata – tunicates, sea squirts

Subphylum Cephalochordata – lancelets

Subphylum Vertebrata (Crainiata) – fishes, amphibians, reptiles, birds, mammals

Subphylum Tunicata

– Sessile, feeding – Mobile, non-feeding

4 chordate traits:Dorsal hollow nerve chord, notochord,pharyngeal gill slits, post anal tail

Subphylum Cephalochordata4 chordate traits:Dorsal hollow nerve chord, notochord,pharyngeal gill slits, post anal tail

Who did the what now?

Neoteny is the retention of juvenile traits in an adult

Specifically, paedomorphosis is the developmental process by which these changes take place

“Well, whenever I'm confused, I just check my underwear. It holds the answer to all the important questions.” – Grandpa Simpson

“Crainiata”Characteristics that distinguish vertebrates:

Extensive skullBackbone - a dorsal row of hollow skeletal elements (vertebrae) which enclose and protect nerve (spinal) cord

Subphylum Vertebrata

Simplest & oldest of all living vertebrates•Sharks (400-500 MYBP)

Most abundant vertebrates (by # & species)

~29,500 living spp of fishes (>482 families)

58% Marine; 1% diadromous fish travel between salt & fresh water

Fishes“I wish, I wish I did not kill that fish” – Homer Simpson

Class Agnatha (jawless fish) Subclass Myxinodea (hagfish) Subclass Petromyzontida (lamprey)

Infraphylum Gnathostomata (jawed fish-cart)Class Chondrichthyes (cartilaginous fish)

Class Osteichthyes (jawed fish-bony) Subclass Actinopterygii (ray-finned fish) Subclass Sarcopterygii (lobe-finned fish)

Fishes

- Cartilage skull - Lack jaws & vertebrae - All Marine (30 spp.) - Scavengers - Produce slime

Subclass Myxinodea(hagfish)

Class Agnatha

- Cartilage skull - Lack jaws & vertebrae - Freshwater & Marine (35 spp.) - Parasites – attach to host - Rasping tongue - Some diadromous

Subclass Petromyzontida (lamprey)

Class Agnatha

- Cartilage skeleton (jaws & vertebrae) - Marine & few FW (750 sp.) - Traces of bone in scales & teeth - Buoyancy via liver – squaline oil - Spiral valve – corkscrew intestine SA:V ratio; compact - Internal fertilization of eggs - Claspers – modified pelvic fins ♂

Class Chondrichthyes(sharks, skates, rays, ratfish)

Placoid scales

Infraphylum Gnathostomata

Skates differ from Rays:Skates have a more muscular tail, two dorsal fins & often a caudal fin, lay eggs

Skates & Rays differ from Sharks:Enlarged pectoral fins that attach to side of head, no anal fin, ventral gill openings, dorsal eyes & spiracles

Ratfishes (Chimaeras):Possess an operculum or gill cover over gill slits, adults have no scales, ♂ clasper on head

Class Chondrichthyes(sharks, skates, rays, ratfish)

Infraphylum Gnathostomata

Shark

Skate

Ray

Ratfish

Early VertebratesEarliest vertebrates - early Cambrian 530 mybp (million years before present)..

Early relatives of agnathans (jawless fishes) first 500+ mybp

Early Fishes

Early Fishes1. Ordovician (505-438 mybp)2. Silurian (438-408)3. Devonian (408-360)4. Carboniferous (360-290)5. Permian (290-240)6. Triassic (240-205)7. Jurassic (205-138)8. Cretaceous (138-63)9. Cenozoic (63-24)10. Quaternary (24-0)

A. Hemicycapsis, B. Pterapsis, C. Cyathapsis,D. Drepanapsis, E. Coccosteus, F. Helodus,G. Cladoselache, H. Raja, I. Chimaera

Early FishesConodonts – (550 mybp) – known from small (<2mm) teeth found in fossil deposits

Large eyes and eel-like bodies, notochordCloser to jawed fishes than lamprey & hagfishes

Early FishesHagfishes - (Subclass Myxinodea)– (550 mybp?) – marine, jawless, eel-like fishes; scavengersSingle nostril, rudimentary eyes, ventral mouth, tongue with rows of keratinized teeth

Early FishesLampreys – (Subclass Petromyzontida) – anadromous or freshwater, lawless, eel-like fishesPredatory & non-predatory formsKeratinized teeth on buccal funnel & tongue

360 mybp

Early FishesOstracoderms – some of the earliest fishesLack jaws, have paired fins, bony armor, cartilaginous skeleton, heterocercal tail(460 mya)

Early FishesAcanthodii – “spiny sharks” – some of the oldest known jawed fishes (440 mybp)Small (<20cm), large eyes, streamlined bodies, dentine-tipped scales

Early FishesPlacoderms – also some of the earliest jawed fishesHeavy bony skeletonsNo special affinities with modern fishes

380 mybp?

PlacodermsMost likely sister group to the combined lineages of Acanthodii, Chondrichthyes, and Osteichthyes; share:

1) jaws with common structure2) Two pairs of paired fins w/ bony girdles3) three semicircular canals in inner ear

Early FishesOsteichthyes – bony fishes – loosely defined groupDefined by common structures and lack of characters that define chondrichthyes

420 mybp

Early FishesChondrichthyes –

What are they Doctor?

Sharks, rays, & skates…

But that’s not important right now…

Early Cartilagenous FishesChondrichthyan fishes most successful measured by historical endurance; ability to survive extinctions

Defined by cartilagenous skeleton mineralized by calcifications (tesserae) and modification of mixopterygia (claspers) in ♂

Early Cartilagenous FishesTwo sister taxa: Elasmobranchii (sharks, rays, skates) & Holocephali (chimeras)

Evidence of Early ChondrichthyansEasier group to define than bony fishes:1) only approximately 850 species2) fossil groups are poorly known

Evidence of Early ChondrichthyansScales & spines from early chond. Identified in Lower Silurian (430 mybp) to Devonian (350 mybp)

Difficult to nail down due to similar morphology among scales & spines of thelodonts & acanthodians (spiny sharks) at this time

Evidence of Early ElasmobranchiiTrue sharks - appeared in middle Devonian (350 mybp); Rays appeared during Jurassic (200 mybp)

Few well preserved specimens; difficult to piece together evolution

Two early forms: cladoselachian & xenacanth sharks

Evidence of Early ElasmobranchiiTwo early forms: Cladoselachian

Xenacanth (350 mybp)

Order CladoselachiformesLacked: claspers, an elongate skull, amphistylic jaw suspension, no anal fin, Had: triangular, paired fins, multicusped teethPredator in marine systems

Jaws…Then & Now

Devonian shark: Snout typically short and rounded; jaws longish and located at the front of the head

Modern sharks: Snout typically longish and pointed; jaws shorter and located underneath the head

Jaws…Then & Now

Long jaws are structurally weaker than short ones and less able to produce a powerful bite

Early sharks may have plucked prey from the bottom or with forceps-like delicacy

Early sharks' upper jaws were fixed to the braincase at both the front and the back (amphistylic) form of jaw suspension

Most modern sharks the upper jaw is fixed to the braincase at the back only (hyostylic) jaw suspension.

Ancient sharks may have been less able to protrude their jaws than modern sharks, reducing their ability to suck prey into their mouths and restricting the size of their food

Jaws…Then & Now

Order XenacanthiformesHad: 2 anal fins, tail diphycercal (pointed)Predator in freshwater systems

Order HybodontiformesAncestral to modern sharksAppeared during Permian (260 mybp)Fed on large, active invertebrates – first with large, sharp teeth

Order ChimaeriformesAppeared during Devonian (350 mybp) with Elasmobranchs

Modern forms during Jurassic (170 mybp)

Modern Cartilagenous FishesMonophyletic groupCommon origin – distinct from bony fishes

Modern Cartilagenous FishesBeyond cartilage have several traits in common….1. Simple box-like cranium2. Upper jaws (palatoquadrate cartilage) not fused to cranium; lower jaw is a single element (Meckel’s cartilage)3. 4-7 internal & external gill openings4. Vertebral column is notochord; becomes supported by calcified vertebrae

Modern Cartilagenous Fishes5. Pectoral & pelvic fins are supported internally by a girdle skeleton; externally by rays (lepidotrichia) of flexible connective tissue6. Basal skeleton on ♂ anal fins – claspers (paired copulatory organs)7. Most have covering of small placoid scales (dermal denticles)

Modern Cartilagenous Fishes

S-U-C-C-E-S-SSuccess due to adaptive characteristics:1) buoyancy2) respiration3) external covering4) feeding5) movement6) sensory systems7) osmoregulation8) reproduction

Buoyancy-no swimbladder-combination of methods to reduce density

Cartilage less dense than bone (1.1 vs 2.0)Large, oil-filled liver (0.8) (water 1.0)Hydrodynamic lift from heterocercal tail & pectoral fins

Respiration3 basic means of respiration1) Two-pump system (like teleosts) – pump O2 water across gills in slow-moving, bottom oriented sharks2) Ram ventilation – push water across gills during swimming; fast-moving sharks3) Spiracles – used to bring water across gills; small round opening precede gills on lateral sides of head – on top of head in rays – almost absent in pelagic sharks

Spiraclesmall round opening precede gills on lateral sides of head

External CoveringAll have placoid scales

Rays – few rows on back; sometimes modified into spines

Sharks – skin overlapping into lightweight, protective coat fast-swimming sharks have channels between scales to minimize turbulence

External CoveringSlow-moving sharks more “armored” - dorsal spines

Rays – barb/sting

Skates – denticles

Teeth are modified placoid scales

FeedingMost are specialized predators – teeth dictate type of feedingTriangular, blade-like teeth –large fish & marine mammals

Long, thin, pointed –whole fish

Rows of small, sharp teeth –small inverts

FeedingFlattened, pavement-like teeth –hard-shelled inverts

Pointed in front/flattened in back –small inverts (grasping and crushing)

FeedingTeeth continually shed & replaced; may loose 30,000 in lifetime

Jaws loosely attached to cranium - can throw jaws or create suction

Large stomach & spiral valve intestine

MovementLarge heterocercal tail

Counter-current heat exchangers in pelagic sharks (Lamnidae)

Pectoral fins (Mylobatidae) – fly through the water

Movement

Homocercal Heterocercal

Sensory SystemsOdor – olfaction detects dissolved chemicals in the water

Low frequency sounds – inner ear and lateral line system; “hear” and “feel” sound waves respectively

Ampullae of Lorenzini – pit organs filled with an electrically conductive gel used for detecting weak electrical currents & magnetic fields

Ampullae of Lorenzini

OsmoregulationOsmoregulators – regulate internal salt concentration approximately 1/3 seawater

Utilize large quantities or organic salts (urea & trimethylene oxide)

Invade marine, estuarine (brackish) and freshwater systems

ReproductionOsmoregulatory and and reproductive systems likely evolved simultaneously;

Long gestation periods of embryos (in egg or ♀) would not be possible without ability to withstand high concentrations of waste

ReproductionUnlike most bony fishes (teleosts) put most energy into relatively small number of large, active, young (ecological term?)

Wide variety of means to this end:egg laying (oviparity) to live bearing (viviparity); all stages in-between